Traditional metropolitan area communications services are based upon technologies such as asynchronous transfer mode (ATM), synchronous optical network (SONET), and Frame Relay technologies, which are optimized for voice communications services. With the increased use of the Internet as a communications medium, non-voice traffic (often referred to as data traffic) is becoming the most prevalent type of network traffic. To meet the increasing demand for data-centric communications services in metropolitan areas, new data-centric metropolitan area networks (MANs) are being built. These new MANs often utilize Ethernet at Layer 2 of the Open System Interconnection (OSI) model to connect nodes within the network (where the OSI model is defined by the International Standardization Organization (ISO)). Ethernet is a popular Layer 2 protocol for use in MANs because of its compatibility with the installed base of end users, its compatibility with the widely used Layer 3 Internet protocol (IP), because of its overall flexibility, and because it is relatively cheap to deploy when compared to other Layer 2 technologies, such as ATM, SONET, and Frame Relay.
Although deploying Ethernet as the Layer 2 technology in MANs has many advantages, the end-user customers, such as businesses, that are targeted to utilize MANs often desire advanced network services such as quality of service (QoS) guarantees, permanent virtual circuits (PVCs), Virtual Leased Lines (VLLs), and transparent LAN services (TLS). Many of these advanced services can be provided by a network that utilizes a Layer 2 technology such as ATM, SONET, or Frame Relay. Ethernet, on the other hand, was not originally designed to provide advanced services and as a result, solutions to customer needs can be more difficult to implement in Ethernet-based networks.
One Ethernet technology that is presently utilized in MANs to provide advanced services to customers is VLAN technology. A VLAN is a group of network devices on different physical LAN segments that can communicate with each other as if they were on the same physical LAN segment. Network devices and their respective network traffic can be mapped into VLAN groups using port-based VLAN mapping, MAC address-based VLAN mapping, protocol-based VLAN mapping, IP subnet-based VLAN mapping, application-based VLAN mapping, and explicit VLAN tagging, or any combination thereof. A widely accepted standard for implementing explicit VLAN tagging in an Ethernet network is defined by the IEEE in its 802.1Q standard. Implementing 802.1Q VLANs involves tagging packets with a Tag Control Information field that identifies the VLAN to which the packets belong. According to the 802.1Q standard, the Tag Control Information field includes a 12-bit VLAN Identifier (ID) field that enables VLANs to be uniquely identified.
FIG. 1 depicts a network 102 that utilizes VLAN technology to connect multiple customers 104 and 106 through a Service Provider Edge Device 112 and two Service Provider Networks 108 and 110. In the example network of FIG. 1, the customers are connected to the Service Provider Networks via an Ethernet-based Service Provider Edge Device 112. In an example network architecture, the customers depicted in FIG. 1 are actually metropolitan service providers (MSPs) that are providing network access to multiple end-users (not shown) and the Service Provider Edge Device and Service Provider Networks belong to large scale network providers, such as the regional Bell operating companies (RBOCs) or long-haul network providers.
Using VLAN technology, a customer, for example Customer A 104 in one location, can connect to another of its locations via the Service Provider Edge Device 112 and the Service Provider Network 108 using a VLAN. As depicted in the example of FIG. 1, the VLAN Identifier (ID) used by Customer A is VLAN ID 100. In operation, the VLAN traffic from Customer A enters the Service Provider Edge Device 112 at port P1 and the input VLAN ID associated with the traffic is used to quickly and efficiently identify the output port for the VLAN traffic. A fundamental principal of known VLAN technology is that the VLAN on which traffic enters a network node is the same as the VLAN on which the traffic exits the network node. In accordance with this principal, the traffic entering port P1 on VLAN ID 100 exits the Service Provider Edge Device through the target output port (i.e., port P5) on the same VLAN ID (i.e., VLAN ID 100) on which the traffic enters the Service Provider Edge Device. VLAN traffic is always kept on the same VLAN because switching traffic to a different VLAN within a network node removes the traffic from the broadcast group to which the traffic was originally associated.
Although VLAN technology works well to provide some advanced services in a MAN environment, VLAN technology has limitations. A significant limitation of VLAN technology that utilizes the 802.1Q VLAN standard is that the length of the VLAN ID field in the 802.1Q VLAN tag is 12 bits. Consequently, any network in which VLANs are deployed is limited to 4,096 unique VLAN IDs (actually, the number of unique VLAN IDs is limited to 4,094 because the value of all ones (0×FFF) is reserved and the value of all zeros (0×000) indicates a priority tag). Because the redundant use of VLAN IDs in the same network should be avoided, the limited number of unique VLAN IDs that are possible using the 12-bit VLAN ID field limits the scalability of a network that utilizes 802.1Q VLANs.
In the example network of FIG. 1, problems of limited scalability and redundant use of VLAN IDs can arise at the Service Provider Edge Device 112 when Customer B 106 wants to forward traffic through the Service Provider Edge Device to Service Provider Network 110 using the same VLAN ID (i.e., VLAN ID 100) as the VLAN ID that is being used by Customer A 104. Because both customers use the same VLAN ID, the broadcast group for VLAN 100 includes ports P1, P2, P3, and P4 (see VLAN table 120). Because the broadcast group for VLAN 100 includes ports P3 and P4, traffic from both of the customers that is received at ports P1 and P2 will be output on the same ports of the Service Provider Edge Device (i.e., ports P3 and P4) with the same VLAN ID. As a result, the customer-specific traffic will be seen by both customers at the far end locations. In order to prevent customer-specific VLAN traffic from being seen by both customers at the far end locations, each VLAN ID within the network should be unique from all of the other VLAN IDs that are used in the network.
One technique that can be implemented to prevent the same VLAN ID from being used by more than one customer within a network involves having the operator of the Service Provider Edge Device (i.e., the Service Provider) administer the assignment of VLAN IDs to the customers. Having VLAN IDs administered by a Service Provider is undesirable because customers typically want the freedom to establish VLANs and assign VLAN IDs independent of their Service Provider.
Even if the assignment of VLAN IDs is administered by a Service Provider, the number of 802.1Q VLANs that can be used within the Service Provider Edge Device cannot scale beyond 4,096 without the redundant use of VLAN IDs. The redundant use of VLAN IDs can be prevented by limiting each customer to some portion of the 4,096 available VLAN IDs, however this limits the ability of the customers to deploy VLAN intensive applications.
In view of the need to provide VLAN-based services using an Ethernet network architecture and in view of the scalability limitations of present VLAN technologies, what is needed is a VLAN technology with greater scalability that can be efficiently and economically implemented.